1. Field of the Invention
The invention relates to cellular telephone systems, and more particularly, to such systems in which data is communicated over a voice channel between a mobile telephone unit and another unit.
2. Description of the Related Art
Data and command signals, as distinct from voice signals, can be sent to and from a mobile telephone unit in a variety of ways and for a variety of purposes. For example, it is well known to use modems with cellular telephones to send and receive computer data and facsimiles. In addition, cellular telephone systems send commands to a mobile unit during call setup and once a call is in progress, e.g. to command the mobile unit to change its transmitter frequency or power level.
A growing demand for mobile electronic productivity tools, e.g. facsimile machines and computers, has created a need for a general purpose data channel that is concurrently available to one or more devices connected to a mobile telephone unit. Currently, when these devices require a data channel, they can initiate and accept calls over the mobile telephone unit using modems to carry the data in the voice bands of the voice channels. However, some users would prefer this data communication to occur in the "background" during normal cellular telephone calls initiated or received by the user of the mobile telephone unit. Furthermore, these users would prefer that several devices' data communication needs be satisfied concurrently during any call.
Similarly, some service providers would prefer to exchange data or commands with a mobile unit or devices connected to the mobile unit concurrently with the normal voice signals carried over the voice channel. For example, some service providers would prefer to send text messages or to remotely monitor various conditions, such as the geographic location of a mobile unit, over a voice channel while a user converses over the channel, without intervention by the user. Problematically, conventional modems do not permit such data exchanges to occur concurrently with normal voice traffic. Providing such capabilities through changes to the cellular control channel or to the voice channel signaling scheme would require regulatory approval and/or changes to installed base station circuitry.